Dia at Sourceforge - tri-platform, open source, 100% free in every way, but still "beta" software. Try adding the shape set at http://dia-installer.de/shapes/building_site/index.html for building-specific shapes, and use for Matrix network diagrams. On Mac requires X11. Not sure about Windows. I had used the program before, adventured into creating "shapes" in Dia, Inkscape and Illustrator to import into Dia. It took a little work, but I have working shapes and created a preliminary map using Dia. With some more tweaking and a larger set of shapes, this could be a very powerful mapping tool.
http://www.dungeonographer.com/index.shtml - cross-platform, Java. Ok, this one is a heavy-lifter. It has a LOT of features, items already available in the editor window, and can generate "Random Dungeons" -- at least some of the base objects are modern-compatible, which helps. However, expect a learning curve, so keep the site open for tutorials and such.
http://evanjacover.com/grumblemapper - cross platform, Adobe Air. A little confusing. Some flaws on the Mac (like turning pieces). I don't think you can move a piece when placed, so that's almost a dealbreaker for me. However, you can add your own images, so some might like it.
http://www.hexographer.com/ - Free, looks good (to a degree) -- somewhat primative outdoor hexagonal mapping. Has upgrade to pro version for some extra power.
Ortelius - Mac platform, vector-based mapping software. This is a professional-grade cartography app, and I'm almost as impressed as I thought I would be from the mini demo reel on their site. The free full-featured version is only watermarked....if you want to get rid of watermarks it will set you back $99 ($49 for students). Here's a sample outdoor map I created. Yummy. Terrific terrain & road-based vector features. (the watermark is worse when you create the pdf than when you're drawing.... so check my test PDF out and decide if it's a dealbreaker for you.)
Paid License Required (from least to most expensive)
RPGTable - Made-for-miniatures & in-game play, $15. This would be a winner if I could only import and scale a background drawing -- at minimum -- so that I could either play on a textured backdrop or pre-made map or use the imported image as a template to draw my own design -- and I'd like to be able to import my own PNG images (with or without translucency) to place on the map. Without the ability to import schematics or maps, add trees or fountains, etc. it's worse than just sketching on a battlemat... But if you wing-it with maps, are tired of fussing with wet erase markers, etc. you might as well wing it in style.